Aqueous polyurethane dispersions are known (cf. Houben-Weyl, Methoden der organischen Chemie [Organic Chemistry Methods], 4th edition, Vol. E20, p. 1659 (1987)). However, unreactive polyurethanes have a number of weaknesses with regard to the high quality profile required by, for example, automotive finishing applications. These weaknesses are primarily due to insufficient cross-linking between the film-forming macromolecules which is manifested in reduced resistance to water, chemicals and solvents and in low-grade mechanical properties. The newer post-curing coating systems which are obtained, for example, by combining isocyanate-reactive resins with blocked polyisocyanate cross-linking agents, afford better properties (cf. J. W. Rosthauser, K. Nachtkamp in Advances in Urethane Science and Technology, K. C. Frisch and D. Klempner, editors, Vol. 10, pp. 121-162 (1987)).
Polyurethane, polyepoxy, polyester or polyacrylate resins and dispersions which are cross-linkable by way of hydroxyl groups are used as resins. The cross-linking agents are blocked polyisocyanates which have optionally been hydrophilically modified. Such systems are known from, for example, DE-A 4,213,527, EP-A 581 211, EP-A 427 028, U.S. Pat. No. 4,543,144, DE-A 3,345,448 and DE-A 2,829,648.
Systems which are considered for paint and coating applications have the property of demonstrating good film-forming properties even at room temperature. If this property is lacking, films having an overall poor quality profile result. This includes, in particular, poor coating flow and low gloss. In some instances, film-formation is also promoted by the addition of solvents and coalescing agents.
The application of powder coatings from the aqueous phase is described, for example, in DE-A 19,523,084, DE-A 19,540,977, EP-A 652,264, EP-A 714,958 and WO 96/37561. A disadvantage of these applications is that during production the extrusion of the binder is followed by a grinding operation which is very laborious and costly. Furthermore, the grinding operation sets limits on the fineness of powders, and this is often noticeable as inadequate sedimentation resistance in the resulting powder suspensions.
It has now been unexpectedly found that useful solvent-free coating dispersions which are based on selected polyhydroxyl components and blocked polyisocyanates, dry in pulverulent manner, are heat-curable can be obtained successfully and economically. The resulting products are useful coating compositions for processing as one-pack products and make it possible to produce coatings having particularly high-grade properties. It is noteworthy that the coating system contains no, or only very small quantities of, organic cosolvents and that coating films having very (good flow properties and a high gloss are obtained which are very highly resistant to water, solvents and chemicals.
A further advantage is the ability to process coatings produced according to the invention on existing wet-coating installations despite the fact that the binders dry in pulverulent manner. With good flow, thinner films are obtained than is the case with conventional powder coating, and spray-flushing the equipment and booths simplifies the cleaning operations in comparison with those of powder coating. Cleaning effort is also less than is the case with solvent-containing finishes owing to the absence of film formation or, compared with two-pack coatings, of cross-linking at room temperature.
The invention overcomes disadvantages of known methods for making water-borne systems. The production of known conventional water-borne coating systems which form films, even at room temperature, either requires considerable quantities of non-ionic emulsifiers, or the systems contain high levels of carboxyl groups which are neutralized with ammonia or amines (ionic emulsifiers). This frequently results in the formation of micro-bubbles during production and application of the coatings, and it is furthermore also the reason for the frequently poor water resistance of corresponding coating films. These coating systems moreover frequently have only a narrow application window (the range of temperature and relative atmospheric humidity at which application is possible) and have a pronounced tendency to form bubbles (boiling). The window is markedly more favourable in the case of binders which dry preferably in pulverulent manner. The surface properties are less dependent on climatic conditions (temperature, relative atmospheric humidity) during application. Greater film thicknesses can furthermore be achieved without the occurrence of boiling due to the evaporation of water.